1. Field of the Invention
The present invention relates to tire treads and more particularly to treads comprising grooves, the latter being provided with closure devices so as to reduce the noise generated by air made to resonate in these grooves during running.
2. Related Art
It is known that as each groove, notably grooves of circumferential overall orientation, enters the contact patch in which the tire is in contact with a roadway during running, air is made to circulate along these grooves. With the roadway, each groove forms a pipe that has two open ends.
The air in this pipe forms a vibrating air column, the resonant frequency of which is dependent on the length separating the two ends of the pipe and therefore on the length of groove affected by contact with the roadway.
This resonance of the air in the grooves has the effect of generating, in a vehicle fitted with these tires, a noise inside the vehicle and a noise outside the vehicle. These inside and outside noises usually correspond to a frequency of 1 kHz or thereabouts, which corresponds to a frequency to which the human ear is particularly sensitive.
In order to reduce such resonance noise, it is known practice (see for example patent document FR2715891) to arrange in each circumferentially oriented groove, or groove with a circumferential overall orientation, a plurality of relatively thin flexible blades or membranes made of rubber compound, each flexible blade or membrane occupying the entire cross section of the groove or at the very least a large proportion of this cross section so as to form a closure device.
Each flexible blade may extend from the bottom of the groove or may be fixed to at least one of the walls delimiting the said groove. Relatively thin here means that each flexible blade is able to flex in order to at least partially open the cross section of the groove under the effect of a flow of liquid notably when driving in the wet. These same blades remain in the position in which the groove is closed when driving in the dry, thus blocking the circulation of air.
By virtue of these flexible blades, the length of the air column in each circumferential groove is reduced by comparison with the overall length of the groove in the contact patch, and this results in a change in the resonant frequency. The shift in frequency is towards resonant frequency values to which the human ear is less sensitive.
Of course, in order to maintain the water drainage function needed for driving on a roadway covered with water, it is necessary for each membrane to be able to flex appropriately under the action of the pressure of the water and thus at least partially open the cross section of the groove in order to allow a sufficient flow of liquid.